Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
Under some conditions, the boost provided by the turbocharger may not be enough under all operating conditions and/or turbocharger lag may occur during transient conditions (such as a tip-in). In some embodiments, an electric compressor may be added to the engine system to increase boost and reduce turbo lag. In particular, the inventors herein have recognized that adding an electric compressor, upstream of the turbocharger compressor in the intake system of the engine system described above, may provide additional boost to the engine without creating excess vacuum that may degrade the compressor. However, the inventors have also realized this positioning of the electric compressor may result in higher pressures in the intake passage than the first exhaust manifold (e.g., scavenge manifold), thereby causing intake air to flow in reverse through the EGR passage and into the exhaust passage via the first set of cylinder exhaust valves (e.g., scavenge exhaust valves). This may reduce a torque potential of the engine while also reducing an efficiency of a catalyst disposed in the exhaust passage, thus increasing engine emissions.
In one example, the issues described above may be addressed by a method, comprising: in response to an electric motor driving an electric compressor positioned upstream of a turbocharger compressor in an intake passage, adjusting a position of a valve in an exhaust gas recirculation (EGR) passage coupled between the intake passage and a first exhaust manifold of a first set of exhaust valves, based on a pressure in the first exhaust manifold. The EGR passage may couple to the intake passage downstream of where the electric compressor couples to the intake passage an upstream of the turbocharger compressor. Further, the electric compressor may be driven by a turbocharger turbine disposed in an exhaust passage coupled to a second exhaust manifold of a second set of exhaust valves. As one example, in response to an inlet pressure of the turbocharger compressor being greater than the pressure in the first exhaust manifold, a controller of the engine may reduce an amount of opening of the valve in the EGR passage. In another example, the controller may adjust the position of the valve in the EGR passage to deliver a requested EGR flow amount in response to the inlet pressure of the turbocharger compressor being the same or less than the pressure in the first exhaust manifold. In this way, by adjusting the valve in the EGR passage based on the pressure in the first exhaust manifold while the electric compressor is being driven by the electric motor, reverse flow through the EGR passage to the exhaust passage via the first set of exhaust valves may be reduced, thereby increasing engine efficiency and reducing engine emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.